support 'numpy' and more general data conversions in 'loading' module

[Marxlp]

Motivation

The data loaded by LoadImageFromFile and LoadAnnotations may be npy file, besides the file format supported by cv2, Pillow etc. And data range may vary. For more details, see #1240

Modification

• mmseg/datasets/pipelines/loading.py: add numpy imdecode_backend, change boolean parameter to_float32 to string parameter to_dtype
• tests/test_data/test_loading.py: Adapt testing script.

BC-breaking

The code setting to_float32 will be affected.

You should change

transform = LoadImageFromFile(to_float32=True)

to

transform = LoadImageFromFile(to_dtype='float32')

For positional format, due the parameter was defined as the last parameter, change

transform = LoadImageFromFile(True)

to

transform = LoadImageFromFile(to_dtype='float32')

Use cases

LoadImageFromFile

results = dict(
            img_prefix = '/path/to/dataset', img_info=dict(filename='image.npy'))
transform = LoadImageFromFile(imdecode_backend='numpy', to_dtype='float32')
results = transform(results)

LoadAnnotations

results = dict(
            seg_prefix='/path/to/dataset',
            ann_info=dict(seg_map='image.npy'),
            seg_fields=[])
transform = LoadAnnotations(imdecode_backend='numpy', to_dtype='int64')
results = transform(results)

Checklist

• [x] Pre-commit or other linting tools are used to fix the potential lint issues.
• [x] The modification is covered by complete unit tests. If not, please add more unit test to ensure the correctness.
• [x] If the modification has potential influence on downstream projects, this PR should be tested with downstream projects, like MMDet or MMDet3D.
• [x] The documentation has been modified accordingly, like docstring or example tutorials.

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[MengzhangLI]

Hi, thanks for your warm-hearted PR!

We would review it ASAP!

[JunMa11]

Hi @Marxlp ,

Thanks for the great contribution.

Does the numpy dataloader support the npy file that contains multiple images? E.g., the shape is (1000, 256,256,3) where 1000 is the number of images.

[Marxlp]

@JunMa11 According to docstring, LoadImageFromFile loads an image from file., therefore, it will treat the npy file with shape (1000, 256, 256, 3) as one image file directly without considering whether the first dimension indicates the batch size. I think it is reasonable, as it's consistent with other file types.

[OpenMMLab-Assistant003]

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